Wiring: Contact vs. Electronic Safety Relays

For many years, wiring of the individual functions on safety relays was a complex, problematic procedure which had a negative impact on the installation process. Imagine the following situation on a machine: A safety gate is intended to prevent random, thoughtless access to a danger zone. Access is only possible once the hazardous movement has been stopped and the machine is in a safe condition, at least within the danger zone. However, the intention is for various drives to be operable at reduced speed, even when the gate is open, for installation and maintenance purposes for example. An enable switch has therefore been installed, which must be operated simultaneously.

If these requirements are to be implemented in practice, so that the operator is protected from potential hazards, a substantial amount of wiring will be needed to connect the individual safety devices. As well as the actual protection for the safety gate, safety relays will also be required for the enable switch, to monitor “Setup” mode, and for the master emergency off/emergency stop function. Reduced purely to the logic relationships, the connections could look as follows:

If this application is implemented using classic contact-based devices, the design will correspond approximately to the diagram below:

 The diagram shows that implementation via contact-based devices produces a result which is not entirely comprehensible; it is also very cost intensive due to the vast amount of wiring involved. In recognition of this  fact, consideration almost inevitably turned to a simpler form of implementation, using logic connections between the safety relays. Thus started the development of a new type of device with integrated connection logic.
 

Microprocessor technology opened up a whole new range of possibilities, as expressed by the predominantly electronic devices in the PNOZelog product series, for example. It laid the foundations for previously unimagined flexibility: One device can now be set for different application areas, another device for different safety functions. Unlike conventional safety relays, these new relays have electronic safety outputs and auxiliary outputs that use semiconductor technology. As a result they are low-maintenance and non-wearing and are therefore suitable for applications with frequent operations or cyclical functions. In addition to the actual basic function, such as monitoring a safety gate or an emergency off/emergency stop function for example, these devices contain a logic block with special inputs, enabling logic AND / OR connections between the devices. An output block with auxiliary outputs and safety outputs completes the safety relay.

The following application example shows how the above example is implemented using electronic safety relays from the stated product series. Compared with a design using contact-based technology, the diagram is much clearer and the amount of wiring is drastically reduced.